The flocculation of suspended matter in water to enhance clarification is a critical aspect in the economies involved in the function of coal mine drainage settling lagoons. Coal production plants generally operate on a closed system basis and, thus, recycle the water used to wash the raw coal. Since such water serves primarily to remove coal fines and clays mixed with the raw coal thereby forming coal liquors, it is critical that the water be relatively clear and contain only minor amounts of suspended solids when it is reused in the washing procedure. If this water is not recycled, fresh water must be substituted and this could cause marked increases in the volume of water employed to efficiently operate the coal production plant.
Previously, the coal industry has employed inorganic coagulants such as lime, alum, ferric chloride, ferrous sulfate, ferric sulfate and sodium aluminate. These chemicals will aid in flocculating suspended solids but have the drawback that they can provide additional quantities of dissolved inorganic solvents which remain in the recycled wash water and cause their own clarification problems.
Another class of flocculants traditionally used in the coal industry are derived from natural products. Starch, starch derivatives and guar gum have been used extensively as flocculants in all types of water systems. However, dosage requirements are high and this can send the treatment cost much higher. Furthermore, the composition of natural products can fluctuate and they can suffer storage problems due to microbiological attack.
Polymeric organic coagulants have also been used as flocculants. These compounds are generally preferred as they produce less waste sludge. This sludge is more compact and these polymers in general will not appreciably affect the pH of the water being treated. High molecular weight polymers such as anionic polyacrylamides have also been used to aid in flocculating suspended solids.
Iron in water can often be found to exist under reducing conditions. In these conditions, iron exists primarily as the ferrous ion (Fe.sup.+2) species. Upon exposure to air, the ferrous ion oxidizes to ferric ion (Fe.sup.+3). The ferrous ion is typically present as the aquo ion, Fe(H.sub.2 O).sub.6.sup.+2, while the ferric ion exists as a hydrated oxide, Fe.sub.2 O.sub.3.n H.sub.2 O. From known solubility data, the Fe.sup.+2 ion is 10.sup.15 times more soluble at pH 7 than is the Fe.sup.+3 ion. Accordingly, iron is soluble in mine water (as Fe.sup.+2). Upon exposure to air (as would be encountered when the water is used to wash the coal), this freshly produced iron species (Fe.sup.+3) becomes almost completely insoluble. These ions can then form deposits on the equipment used to wash the coal and interfere with the efficiency of the wash process.